The fine regulation of the thiol-disulfide status is one of the most important basic requirements of biological metabolic powers. The central regulation element within this system is the tripeptide glutathione, which reaches relatively high concentrations (up to 10 mM) intracellularly in reduced form. In addition to glutathione, proteins bearing thiol groups intracellularly and in particular in cell membrane-bound form are further important units of the thiol-disulfide status of each cell.
The metabolism of the disulfide cleavage and thiol group formation regulated by various enzyme classes is in its entirety indispensable for every normal cell function due to the variety of its biological functions, inter alia in cellular growth and differentiation processes including programmed cell death and cell protection and detoxification mechanisms. Disturbances in this system and changes in the concentration of thiols lead to serious cellular functional disorders, which only remain locally restricted in the isolated case, but as a rule adversely affect the entire organism.
It was possible to demonstrate the involvement of a disturbed thiol-disulfide status in acute and chronic disorders in a large number of investigations.
Thus, for example, marked changes in the thiol metabolism were detected in certain nerve cells in neurodegenerative disorders such as Parkinson's disease (Brain Res Rev 1997; 25:335-358). There are clear indications that as a result of this metabolic disturbance an increased death of the nerve cells substantially responsible for the symptomatology of the disorder occurs in functionally impaired areas of the brain, the basal ganglia (Ann Neurol 1994; 36:348-355).
Decreased glutathione levels or a decreased intracellular glutathione content was furthermore found in the course of vascular disorders and their sequelae—arteriosclerosis and cardiac infarct—in the endothelial cells lining the vascular inner wall (Med Sci Res 1998; 26:105-106).
Pulmonary disorders which are accompanied by a turnover of the lung tissue are regularly connected with a glutathione deficit in the tissue. In such pulmonary fibrosis, the degree of severity of the disorder proceeds in parallel to the thiol loss (Clin Chim Acta 1997; 265:113-119). Severe inflammatory pulmonary disorders, investigated in the example of adult acute respiratory distress syndrome, are accompanied by a dysregulation of the thiol metabolism of the inflammatory cells (granulocytes) involved (Chest 1996; 109:163-166).
Immunocompetent defense cells of the bronchial system (alveolar macrophages) of smokers and patients with chronic obstructive airways diseases exhibit, according to our own investigations, a severe cellular thiol deficit. The degree of the disturbance of the cellular thiol status in this case correlates directly with restrictions of the lung function (Free Radic Biol Med 2000; 29:1160-1165).
In recent years, increased references to a damaged thiol metabolism have been found in chronic kidney diseases (Ren Fail 1998; 20:117-124), anemia (Br J Haematol 1995; 91:811-819), immature newborn children (Pediatr Pulmonol 1995; 20:160-166), noise-related hearing loss (Brain Res 1998; 784:82-90), inflammatory intestinal disorders (Gut 1998; 42:485-492) and in diabetes mellitus (Metabolism: Clinical and Experimental 1998; 47(8):993-997).
Extensive investigations on the importance of the glutathione metabolism in virus infections demonstrated both a relatively poor prognosis of thiol-deficient cells, based on a damaged cellular defense, and an antiviral function of the glutathione inhibiting virus replication (Proc Natl Acad Sci USA 1997; 94:1967-1972). The cellular human immune system, consisting of the white blood cells granulocytes, lymphocytes and monocytes is a system reacting particularly sensitively to a disturbance in the thiol metabolism.
Minimal changes, in particular losses of cellular glutathione, can induce a cascade-like program for the self-destruction of the cell, programmed cell death (apoptosis) (FASEB J 1998; 12:479-486). The thiol-disulfide metabolism acts here as a central control member of an intact immune system, without which the organism would not be viable.
Our own investigations showed that in particular under the conditions of a high-grade restricted kidney function and kidney replacement therapy necessary as a result in the form of hemodialysis or peritoneal dialysis, the cellular thiol-disulfide metabolism is severely disturbed. This disturbance results, inter alia, in an extensive loss of normal cell functions, such as the phagocytic ability of peritoneal macrophages or the activatability of lymphocytes. In these patients, in addition to the existing local immune deficit, which is characterized by frequent infections of the abdominal cavity, a markedly decreased immunological defense with increased general susceptibility to infection are regularly found. Functional disturbances and a decreased activatability of the lymphocytes and macrophages, and imbalances of the immunoregulatory cytokines are in particular described here (Immunobiol 1999; 200:62-76).
The correction of a disturbed thiol metabolism thus attains fundamental importance as a basic therapy in the treatment of a large number of disorders of different origin, but in particular under the conditions of a necessary kidney replacement therapy.
α-Lipoic acid is up to now being employed with relative success as a neuroprotective substance for the treatment of neurotoxically related paresthesias in diabetic polyneuropathy (Diabetologica 1995; 38:1425-1433, Diabetes Res Clin Pract 1995; 29:19-26, Diab Care 1999; 22:1296-1301, Drug Metab Rev 1997; 29:1025-1054, DE 43 43 592 C2). The use of α-lipoic acid in further neuronal disturbances including tinnitus and sudden deafness is moreover known from DE 44 47 599 C2 and EP 0 530 446 B1.
The cytoprotective mechanism of action is based here, in addition to the influencing of the sugar-dependent protein modification (protein glycosylation) and a decrease in the neurotoxic ketone body genesis, finally on the antioxidative function of the α-lipoic acid and its metabolites (Free Radic Biol Med 1995; 19:227-250).
This cell protection function was particularly investigated under the aspect of the prevention of the oxidative turnover of essential unsaturated fatty acids. Such an inhibition of the lipid peroxidation is, in addition to the use of α-lipoic acid as a neuroprotective, the basis for an application as a hepatoprotective medicament in various intoxications and liver disorders (Biochemistry 1998; 37:1357-1364).
Moreover, it was possible to show that α-lipoic acid inhibits the replication of the HI virus at different stages of development and thus could counteract progression of the AIDS disease. It was only possible, however, to transfer the results of these laboratory experiments restrictedly to clinical studies (FEBS-Lett 1996; 394:9-13). The same applies for the demonstration of an antiinflammatory function of the substance for the insulin-producing islet cells of the pancreas (Agents Actions 1993; 38:60-65).
In EP 0 812 590 A2 and EP 0 427 247 B1, the use of α-lipoic acid as a cytoprotective, analgesic and as a medicament in inflammatory diseases is disclosed.
The antioxidative properties of α-lipoic acid are based, in addition to the ability to form chelates with metal ions and directly to eliminate radicals, in particular on the function as a strong reductant. In order to carry out this reaction intracellularly, α-lipoic acid itself must be present in reduced form, as dihydrolipoic acid. The conversion of (disulfidic) α-lipoic acid by means of reduction to the dithiol form of dihydrolipoic acid uses, for its part, reducing equivalents, this process being catalyzed, inter alia, by the enzyme glutathione reductase (Gen Pharmacol 1997; 29:315-331). This is obviously the cause of the hitherto unsatisfactory action of the substance with respect to the thiol restitution.
DE 44 20 102 A1 describes a pharmaceutical combination of α-lipoic acid and cardiovascular-active substances, in particular an organic nitrate, a calcium antagonist, ACE inhibitor or oxyfedrine. The pharmaceutical combination should be employed for the treatment of cardiovascular diseases and diabetes-related diseases.
Ambroxole, i.e. trans-4-(2-amino-3,5-dibromobenzyl-amino)-cyclohexane hydrochloride is employed as a mucolytic medicament in various administration forms in lung and bronchial diseases (WO 96 33704, GB 2239242, WO 01 05378). Moreover, the use in hyperuricemia is known from DE 35 30 761. The action of ambroxole as a mucolytic is based both on a stimulation of the surfactant production of the bronchial cells and in particular on the ability to eliminate free radicals (Respir Med 1998; 92:609-23). This antioxidative activity of the substance based thereon was mainly demonstrated on pulmonary cells (Pharmacol 1999; 59: 135-141) but also in the context of inflammatory mechanisms (Inflamm Res 1999; 48:86-93). Furthermore, it is known that in vitro regulatory enzymes of the glutathione metabolism are directly influenced and per-oxidative processes are inhibited by the addition of ambroxole in high doses, and peroxidative processes are inhibited (Arch Vet Pol 1992; 32:57-66).
Angiotensin-converting enzyme inhibitors (ACE inhibitors) are employed with great success in the treatment of a wide range of cardiovascular diseases. The cause of the hypotensive action utilized here is based on the inhibition of the conversion of angiotensin I to angiotensin II. Moreover, ACE inhibitors were also described as effectors of the glutathione metabolism. In addition to investigations on relevant effects in cardiovascular and vascular disorders (J Cardiovasc Pharmacol 2000; 36:503-509) general regulation principles were investigated (Clin Nephrol 1997; 47:243-247). The actions of ACE inhibitors bearing SH groups, such as, for example, captopril (1-[(2S)-3-mercapto-2-methylpropionyl]-L-proline) from SH-free ACE inhibitors such as, for example, enalapril (1-{N—[(S)-1-ethoxycarbonyl-3-phenylpropyl]-L-alanyl}-L-proline) are to be distinguished here. The former react directly as radical scavengers antioxidatively while SH-free ACE inhibitors are primarily not able to do this. Common to both groups is the influencing of the glutathione redox cycle via the regulation of the glutathione reductase and glutathione peroxidase, and furthermore superoxide dismutase (Am J Physiol Regulatory Integrative Comp. Physiol. 2000; 278:572-577).
Accordingly, there is a need to make available novel medicaments containing thiol-reactive substances for the improved stabilization of a damaged thiol-disulfide status, in particular in the case of renal insufficiency in kidney replacement therapy, and for the restitution of the functional losses caused thereby.